Metro says ridership levels will become “unmanageable” by the end of the decade, and riders who take up two seats while paying for just one are making the problem worse. Seat hogs would rather give the seat next to them to their briefcase or umbrella or smelly sneakers than let other riders use the seats they are paying for.

One New York man, who wisely keeps his name to himself, created a website called Seathogs.com, which posts pictures of extra-seat-stealers from transit systems around the world. He told the Post, “Seat hoggers and people being rude in public has kind of reached a boiling point, with the economy bad.”

The man blames the “sense of entitlement among certain passengers.”

But why? Why do some riders think they have the right to two seats or more when others are standing? And what’s so bad about having someone sitting next to you, anyway?

On New York’s subway, seat hogging is actually against the law. That may seem to make sense, but it has its downside. A few months ago, the New York Times ran an item on a man pulled off a train for taking up two seats -- on a nearly empty train at 1:30 in the morning.

The man said he told the police officers, “‘I understand what you’re doing, but in what way was I obstructing someone else from sitting down?’ The officer said, ‘Look, man, this is just what we’re doing tonight.’”

The New York offense carries a $50 fine -- and when cities are hard up, they look for ways to nab offenders to run up the revenue.

There’s no such law here. The Post says WMATA prefers “to rely on the civility of Washingtonians” -- perhaps forgetting JFK’s maxim about D.C. being “a city of Southern efficiency and Northern charm.”